Hospitals were swamped with emergency patients, while firefighters and police were called on to help paramedics respond to thousands of calls after the conditions caused breathing problems for Victorians.

Ambulance Victoria emergency operations general manager Mick Stephenson said two people died in Melbourne's western suburbs after reporting respiratory symptoms.

"We do know of those two cases and there will be other people who died in Victoria yesterday as well," he said.

"Whether or not they're related to this phenomena, we don't yet know. We'll have to review each incident and, as I say, in the fullness of time we'll understand whether or not they are related."

Mr Stephenson said one of the patients had waited at least 30 minutes for an ambulance.

An internal review was underway into the circumstance of the two deaths, he added.

There were 2,000 calls to triple-0 for ambulances between 6:00pm and 11:00pm — nearly seven times more than usual.

'We ran out of Ventolin puffers at one point'

St Vincent's Hospital, in the inner-Melbourne suburb of Fitzroy, was overflowing with emergency patients and ran out of Ventolin puffers.

"The emergency physicians say not only have they never seen anything like that before, they've never seen so many people arrive at one time all suffering the same condition," hospital spokeswoman Kathy Bowlen said.

"We were considering opening a second emergency department … we ended up doing that using the day procedures area.

"We ran out of Ventolin puffers at one point."

The hospital enacted its Code Brown plan to free up beds for extremely sicks patients during external emergencies.

Andrew Walby, the hospital's director of emergency medicines, said it was the first time, to his knowledge, a Code Brown had been called at the hospital in at least five years.

"We had one patient who was unable to manage to maintain their own breathing and ended up requiring to be put on a ventilator and go up to our intensive care unit," he said.

Health Minister Jill Hennessy said there would be a review into how hospitals and the ambulance service handled the emergency.

"No doubt there will be lessons to learn — there always are in these cases," she said.

"Particularly, in this season, if you've got allergic reaction, keep up your preventer medication [and] talk to your doctor about the best way to use your medication," he said.

"Also, if you start to have symptoms, that's where your blue puffer comes into play."

More than 400 children were treated at the Royal Children's Hospital, and more than 60 patients were still waiting to be seen at 8:30am.

"The most seriously injured or sickest children are always seen first at [the emergency department] so children with less urgent conditions currently face long wait times to see a doctor," a statement on the hospital's Facebook page read.

Yarraville's Carnovale Pharmacy — the only 24-hour pharmacy in the inner west — was extremely busy overnight and ran out of asthma medication.